The present invention relates to a foamable composition and a coaxial cable having an insulating foam layer. More particularly, the present invention relates to a foamable composition and a foam suitable for a high frequency coaxial cable, particularly a coaxial insulating cable preferable for a trunk line of CATV and a feeder antenna of a portable communication device such as cellular phones, and to a coaxial cable having an insulating foam layer.
Conventional foamable compositions used for the production of a feeder antenna of portable communication devices contain an olefin resin, such as polyethylene and polypropylene, and a so-called chemical foaming agent as a nucleator, such as 4,4xe2x80x2-oxybisbenzenesulfonyl-hydrazide (OBSH) and azodicarbonamide (ADCA). Conventional foams have been obtained by foaming the above-mentioned foamable compositions with a foaming agent such as various inert gases and hydrocarbon gas. In addition, the use of a fluororesin powder or boron nitride powder as a nucleator to obtain a foam having high electric property has been known.
While a feeder antenna of a portable communication device has been increasingly miniaturized and a higher frequency band has been employed in recent years, a coaxial cable having the above-mentioned conventional foam as an electrically insulating layer often fails to show required attenuation property due to its insufficient electric property.
The electric property of a foam has been so far improved by mainly studying nucleators and foaming agents. However, the present inventors took note of polyethylene to be foamed. As a result, it has been found that when a fluororesin powder or a boron nitride powder is used as a nucleator, the use of a specific polyethylene mixture consisting of a low density polyethylene and a high density polyethylene results in the production of a foam having unexpectedly high electric property. In addition, a small-sized coaxial cable having this foam as an electrically insulating layer has been found to exhibit superior attenuation property at a high frequency band.
The present invention is based on the above-mentioned new finding and aims at providing a foamable composition preferably used for a coaxial insulating cable having superior attenuation property, that is suitable for a feeder antenna of a portable communication device and the like, and a coaxial cable having an insulating layer made from the composition.
Such object can be achieved by the following foam able composition and coaxial insulating cable of the present invention.
The foamable composition comprises a base polymer comprising a polyethylene mixture consisting of a low density polyethylene and a high density polyethylene, and a fluororesin powder and/or a boron nitride powder as a nucleator, wherein the high density polyethylene has an SR smaller than that of the low density polyethylene and an MFR (melt flow rate) greater than that of the low density polyethylene, the high density polyethylene is contained in a proportion of not less than 50 wt %, and the base polymer has a shear viscosity (temperature: 170xc2x0 C., shear rate: 1216 secxe2x88x921) of not more than 3150 poise.
The coaxial insulating cable comprises an electrically insulating layer made of a foam having an expansion ratio of not less than 70% and obtained by foaming the above-mentioned foamable composition using an inert gas as a foaming agent.